Support RGBM images

[Philip Rideout]

On the Filament team we often squeeze HDR out of PNG files by repurposing the alpha component. I don't think HDRView currently supports this, I could try implementing myself with a little guidance.

[Wojciech Jarosz]

I'm happy to look over a pull request if you decide to add this. To get you started you could take a look at HDRImage::load() in HDRImageIO.cpp. I think the tricky part will be to detect how the alpha channel of the PNG should be treated. If there is an accepted standard extension for RGBM PNG images, then you could use that. If not, this might be more tricky as HDRView doesn't currently allow the user to identify the file format -- It tries to guess the format automatically based on whether it can read the files (e.g. see isSTBImage in HDRImageIO.cpp).

[Philip Rideout]

Come to think of it, this isn't well standardized, so I think we can close this issue. Thanks for hearing me out though!